Among the given options, the largest monkey is the Gorilla (option 3). Gorillas are the largest primates and belong to the ape family. They are native to the forests of central Africa. Gorillas have a strong and muscular build, with adult males weighing up to 400-500 pounds and standing at a height of around 5.5 to 6 feet tall when standing upright.

Option 1, Spider monkeys, are medium-sized monkeys found in the tropical rainforests of Central and South America. They are known for their long limbs and prehensile tails, but they are not as large as gorillas.

Option 2, Baboons, are also large monkeys but smaller than gorillas. They are mainly found in Africa and Arabia.

Option 4, Howler monkeys, are known for their loud vocalizations, but they are not the largest monkeys. They are medium-sized monkeys found in the forests of Central and South America.

In conclusion, the Gorilla is the largest monkey among the given options.